1. Field of the Invention
The present invention relates to an apparatus for controlling a hybrid vehicle. More specifically, the present invention relates to an apparatus for controlling a hybrid vehicle to reduce its fuel consumption.
2. Description of the Prior Art
An electric vehicle is one of the most effective means of transportation by virtue of its freedom from pollution as compared with vehicles employing other conventional prime movers. However, an electric vehicle suffers from various disadvantages; a long period is required to charge the battery, and both running distance and running time are short. In view of the foregoing, a hybrid vehicle (an electric vehicle with an internal conbustion engine) has been proposed and put into practical use, eliminating these disadvantages without losing the advantages of an electric vehicle. Naturally such a hybrid vehicle consumes fuel in an engine running mode and consumes electric power in a motor running mode.
An automobile having the smallest possible fuel consumption has been desired. It has also been desired to purify exhaust gas from the automobile since gas from an engine is one cause of air pollution. It has been well-known that exhaust gas from an engine is purer when engine efficiency is better.
On the other hand, an automotive internal combustion engine requires a wide range of torque and speed so as to be adaptable to conditions of use, such as constant speed operation, acceleration, hill climbing and the like. Nevertheless, each internal combustion engine has a region in which its fuel consumption is minimized. An automotive internal combustion engine operates in that region when climbing hills or accelerating. Therefore, it is conceivable to minimize engine capacity and therefore minimize maximum power, so that the automobile is normally operated in its region of minimum fuel consumption. However, with only such an engine, it is impossible to attain a required driving torque which will suffice in all running conditions. Therefore, the inventors of the present invention previously proposed an improved hybrid vehicle wherein the engine is always operated in its most fuel-efficient region, and driving torque deficiencies are made up by an electric motor, whereby overall fuel consumption as well as overall energy efficiency of the vehicle is enhanced.
However, previous hybrid vehicles required an electric motor having a power sufficient to satisfy all speed characteristics and torque requirements for all running conditions. In such cases, motor and engine efficiency is poor in a low load condition such as normal operation, and saving energy is difficult. On the other hand, it is well-known that an electric motor can be used as a generator during braking, thus saving energy. However, since the motor in a previously proposed hybrid vehicle has a large continuous rating, it is impossible to so use the motor. More specifically, when a motor with a large continuous rating is used in a hybrid vehicle, the motor does not work well as a generator, because when the vehicle speed drops, the motor will not produce a sufficiently high voltage. It is also conceivable to employ a voltage boosting means such as a transformer, but even this cannot provide a sufficiently high voltage without having extremely poor efficiency. Since a vehicle is operated at speeds between 0 km/h to 100 km/h, it could very often happen that the speed would not be high enough to provide enough voltage to regenerate its battery. Accordingly, a hybrid vehicle is not efficient enough. The size and cost of a motor/generator increase approximately as the square of its capacity, and a motor/generator that is capable of coping with all running conditions becomes large and extremely expensive.